An electrocardiogram (ECG) measurement consists of measuring a small, approximately 5 milli-Volt, signal produced by the heart superimposed upon a relatively large, approximately 300 mV, low-frequency potential produced by the skin-to-electrode interface. The large low-frequency potential is called baseline wander. It is desirable to remove the baseline wander without altering the ECG signal.
The American Association of Medical Instrumentation (AAMI) has specified two cases for the removal of the electrode offset. The first specification is for the case of ECG monitors. In this case, it is more important for the ECG signal to remain visible on the screen than to make a diagnosis based on the precise measurements of the ECG waveform. Thus, the monitor specification requires the frequency response to be only as low as 0.5 Hz. The most typical filter for an ECG monitor is a single pole high pass filter having a 3 dB corner of 0.5 Hz.
The second specification is for the case of diagnostic ECG measurement. This specification requires a passband down to 0.05 Hz so the high pass filter causes only minimal distortion of the ECG signal. This maintains a high degree of accuracy allowing for the diagnosis of a heart.
Typically, the distortion that is created by a single-pole high-pass filter is due to its nonlinear time delay. A single-pole 0.5 Hz high-pass filter can be greatly improved by giving it constant delay. Alternatively, the corner frequency can be decreased, increasing the filter's susceptibility to baseline wander.
The effect of nonlinear time-delay distortion on the diagnosis of an ECG signal is most pronounced with respect to a calculation of ST segment elevation or depression. This calculation is based on the difference in voltage from the PQ segment just before the QRS and voltage of the ST segment just after the QRS. Solutions to this problem have in the past consisted of either decreasing the frequency of the high pass filter as shown by the AAMI specification of diagnostic ECG instrumentation for 0.05 Hz, or adding a minimum of 2 seconds of delay in order to perform a constant delay 0.5 Hz high pass.
For the foregoing reasons, there is a need for an ECG high-pass filter which has decreased susceptibility to baseline wander, yet provides an accurate representation of the input ECG signal for monitor and diagnostic purposes.